Heat transfer unit



April 20, 1954 s. N. COATES HEAT. TRANSFER UNIT Filed June 30, 1952INVENTOR. svo/vgv M 00A 755 Patented Apr. 20, 1954 UNITED STATES PATENTOFFICE HEAT TRANSFER UNIT Sydney N. Coates, Seattle, Wash.

Application June 30, 1952, Serial No. 296,409

4 Claims.

The present invention concerns a heat transfer unit by which heatgenerated by an electric resistance element of the rod type istransferred to air in an ambient space. That air may be moved in acurrent through a duct, as by a fan, or it may simply be air in thegeneral space surrounding the heat transfer unit.

Primarily it is an object to provide a heat transfer unit of the generalnature indicated, which has a number of important advantages over knownheat transfer units which employ rod-type electric resistance elements,in that the newer type of'this invention is explosion-proof, is cheaperto manufacture and more durable, is electrically and thermally moreefficient, operates at lower sheath temperatures, and can operate at theusual or even at higher voltages without danger of electrically chargingthe unit, permitting more than one such element to be connected inseries.

More specifically, it is an object of the invention to utilize arod-type electric resistance element as the heat source, withoutmodification or addition in any way, but to transmit the heat sogenerated across a closed air space to a much larger heat-radiating,finned housing manufactured as a separate component, for transfer inturn to the air, and to electrically insulate the rod-type element,including its sheath, from the housing in such way that electricaldischarge from the rod-type element to the housing, even at voltages sohigh that the sheaths of the rodtype elements might under previouslyexpected conditions become charged, is virtually impossible. Thereby itbecomes possible to achieve many advantageous objectives, as will appearhereinafter.

It is also an object of the invention to provide a heat transfer unit ofthe general type indicated, and a method of making the same, in whichthe fins normally used as the primary heat transfer surfaces may bemounted and held upon the tubular unit which surrounds, encloses, andpicks up the heat from the rod-type resistance element, yet needs nomeans to mount them, such as welding, brazing or the like, and can bereadily and permanently fixed and held in position, and also in suchmanner that high temperatures, and the expansion accompanying the same,will operate to hold the fins even more tightly in place, rather than,as heretofore when they were brazed to the rod-type elements, to breakthem loose.

The present invention consists of a tubular metallic housing surroundingand spaced by an appreciable distance from a normal rod-type electricresistance element which extends through the tubular housing, the twobeing mutually supported. by end closures or caps of dielectric andrefractory material, so that the enclosed space within the tubularhousing is a closed air space,

across which heat is transmitted from the rodtype electric resistanceelement to the tubular housing, and the tubular housing is provided withencircling fins, mounted upon it, and capable of radiating the heatwhich they pick up from the tubular housing. A heat transfer unit ofthis nature is of appreciably larger diameter than the known type inwhich the fins contact directly the rod-type electric resistanceelement, and thereby the radiating surface is greatly increased, thesheath and the housing temperatures can both be kept low, and a smallnumber of units, operating at high efficiency and yet at lower actualhousing temperatures, will produce heating the equivalent of a muchlarger number of the old type heat transfer units.

The invention comprises the novel heat transfer unit in its entirety,the special finned tubular housing as an element or component thereof,and the process by which the fins are assembled and held in place onthat tubular housing.

The invention is disclosed in the accompanying drawings, and will behereinafter described and claimed in the form which presently ispreferred by me, but it will be understood that various changes may bemade in the form, character and arrangement of the parts withoutdeparting from the spirit of the invention, and that no restriction isto be implied other than is incorporated within the claims.

Figure 1 is a longitudinal axial sectional view through such a heattransfer unit in its completed form, one end only being shown and theunit being of indeterminate or indefinite length, to suit conditions,and Figure 2 is a transverse half-sectional view through this unit, theviewpoint being indica ed by the line 22 in Figure 1.

Figure 3 is an isometric view of several such units, with some partlybroken away, installed in a typical form in a duct, also broken away toshow the interior.

Figure 4 is an axial sectional view through such a housing and fins inthe process of mounting and fixing the fins on the tubular housing.

Rod-type electric resistance elements are known, and it is the intentionto take such known units and to utilize them in a different combinationand relationship, but without change in the rod-type electric resistanceelements themselves. However, for purposes of clarification such anelement is shown in section in Figure l, and consists of a coiled wire 9of material possessing a high electrical resistance, mounted upon and inelectrical contact at its ends with pins Bil, the whole closely enclosedwithin a metallic sheath 3!, which is filled with a granular dielectricmate rial indicated at 92, magnesium dioxide being usually employed. ihepin projects from the end of the sheath 9| and is threaded at 93 for therod-type the reception of binding nuts 94, by which the terminal 80 ofan electrical conductor 3 is electrically connected.

Heretofore such rod type electric resistance elements have had finsmounted directly upon them, and held in place by brazing the fins to theelements sheath. The normal high temperature of the sheath is lowered byconduction into the fins, and so long as the heat can be removed, byconvection or radiation or both, such prior units could operatereasonably satisfactorily. However, if the finned unit of the known typeis not adequately cooled-to do which usually requires the blowing of airpast it continually with a fan-its temperature may rise so high that thefins become detached, and the element, which is not then adequatelycooled, burns out. Such prior units, therefore, are usu ally employedonly in installations that include a circulating fan. This adds to thetotal initial cost, weight, complexity, and operating complications ofthe installation as a whole.

Moreover, the temperature in such prior units was high at best, andhigher in the event of failure of the fan or blockage of air ci ulation,and always created an explosion hazard, both from high temperature andfrom the danger of an electrical spark.

Electrically, the small gap between the sheathed coil 9 and its sheathill, even though reinforced by dielectric granular material couldwithstand safely only a relatively low volt-- age; also, the small gapbetween the pin and the end of the sheath ill, in open air, peli itteddischarge to the sheath. if the voltage is only reasonably high, andthese factors 1. ted the voltage permissible, wherefore such L its couldnot be connected in series, using a high volta e, and even at lowvoltage there was danger of electrical discharge from the coil or theterminal to the sheath, and of consequent charging of the sheath andeverything in conductive relation therewith.

Mechanically, the fins in the prior units had to be mounted upon andbrazed to the sheath, which is small and subject to relatively greatranges of expansion and contraction, due to the wide range oftemperature to which it is subjected. This was a difiicult and somewhatoi:-

pensive job, and not infrequently the hard solder holding the finsmelted and the fins came loose, and hence lost much of their value toremove heat from the sheath. This mechanical breakdown occurred usuallywhen the units becarne overheated from lack of an adequate supply of.cooling air passing over the units although sometimes in othercircumstances-and the danger of such a breakdown required that fans andducts be used with such prior units, and by so much limited theirusefulness and increased their initial cost, weight, complications, andoperating expense.

According to the present invention these known rod-type, electricresistance elements are employed in somewhat the same way, but nowelements are taken as they are completed by the manufacturer and areincorporated without addition or change in the heat transfer unit as awhole. Now, according to this invention, such a unit is supported withina tubular housing 1, preferably one made of metal of good thermalconductivity, which housing surrounds, extends lengthwise of, and isspaced appreciably atv all points from the sheath 9!. The fins 2 aremounted, according to the present invention, not

upon the sheath 9!, but upon the housing I, which they encircle. Thesefins 2 may be in the form of individual disks, of whatever marginalcontour desired, or they may be in the form of a continuous helix.Separate circular disks are il lustrated herein, but the principle isthe same in any case; they constitute metal fins which are mounted onand in heat-transfer engagement with the housing, and they projectoutwardly from the housing. The manner of mounting them, and ofretaining them, will be explained hereinafter.

The tubular housing I, with its fins 2, is closed at its ends by acentrally axially bored dielectric closure member 3, preferablyshouldered at 30 to receive the end of the housing i, its centralaperture or bore constituting a support for the rodtype electricresistance element and its sheath 9| Preferably the sheath terminateswithin the dielectric closure 3, insulating washers e1 complots thefilling of the bore in the closure to prevent discharge from theprotruding pin iii} to the sheath iii, and a washer of insulatingmaterial overlies the end of the bore and is held in place by a cap 99.This cap as serves to anchor the rod-type element in place, inconjunction with the binding nuts 94.. The dielectric closure cap 3likewise be shouldered at 35 for mounting within an aperture of a ductl. The rodtype resistance elements may operate more nearly at theirnormal temperatures, without fluctuation, and so their electrical andthermal efficiency is greater, yet being enclosed within a completelyenclosed dead air space, any electrical break:-

' down that could charge the sheath til (now extrenieiv unlikely becauseof the thorough dielec insulation at all points) cannot possibly createa hazard to persons contacting in-- stallation, and the absence of thepossibility of electrical discharge, coupled with low housingtemperature, eliminates any explosion or fire hazard. Instead of a gapof a small fraction of an inch between the resistance coil e and thesheath 9!, and between the pin iii} and the end of the sheath, whichprecludes he use of high voltage when the sheath is in contact withgrounded parts, the sheath is now mounted in the large dielectricclosure 3, the pin all is thoroughly insulated electrically from sheath,and the sheath is spaced by a large distance from the housing i, so thatthere is little or no danger of discharge across to the housing, evenshould the sheath become charged, which, as has been indicated, is nowextremelyunlihely. Now several such units may be conne ted in series,with high voltage current supply, without danger of the housings.becoming charged electrically.

It would be difficult to express. any formula for determining thespacing between the sheath 9| and the tubular housing I, since variousfactors, diii'ering in different installations, may govern. In general,there should be a rather large air space, radially, within the housing.The larger the air space the greater the. surface area of the housingand the lower its surface. temperature, in general. Of course, thisspacing must not be so great that the transfer of heat from the rod-typeelement to the housing is impeded to the extent that the element becomesoverheated, and is in danger of burning out, or to the extent that theunit as a whole becomes excessively bulky. The optimum air cap i mallenou h that heat is rcmoved from. the. rod typcelcment fast enou h okeep it reasonably 0001. yet lar e. enough to avoid the development inthe housin of an excessivel 5. high temperature. Purely by way ofillustration, a unit wherein the air gap is -7 inch all around produceda housing temperature of about 1 F. when the sheath temperature of therod-type element was 750 approximately.

As a result of the enlargement of the heat transfer unit as a whole, notonly is the surface area of the housing increased greatly over thesurface area of the sheath 9!, but so also is the area of the fins 2increased greatly over the area of the fins as mounted heretoforedirectly on rodtype resistance element. The much larger area cantherefore dissipate the same amount of heat in a given period of time ata much lower temperature, and a few units will accomplish what beforerequired many units. No heat is lost; as much as is generated by a givencurrent is of necessity transmitted to the housing and thence to thesurrounding air, and the omission of fans is not harmful.

Further according to the present invention, each housing I, being madeof sheet material such as aluminum, may have a relatively highcoeflicient of thermal expansion, and yet be soft enough to bedeformable with reasonable readiness. The fins 2, however, arepreferably of somewhat harder material, but whether harder or not, theyshould be of material having a lower coeflicient of thermal expansion.These relationships make possible a special form of assembly, and assurethe retention of the fins regardless of thermal changes. I

Initially, then, the housing I is of slightly smaller external diameterthan the interior margins or openings in the fins 2, as shown in Figure4. The fins are disposed in properly spaced relationship, encircling butnot necessarily contacting the exterior of the smaller housing I. Theends of the housing are closed as by a closure member 4, and by means ofa conduit at a pressure fiuid, whether pneumatic or hydraulic isunimportant, is admitted to the interior of the housing I, and thepressure is such that the tubular housing I is expanded. When it expandsuntil it contacts with the interior margins of the fins 2, that portionin contact cannot expand farther, but continuation of or addition to thepressure within the housing expands the tube in the portionsintermediate the fins until it bows outwardly, as shown in Figure 1.Upon removal of the pressure fluid this expansion or distortion of thehousing I persists, and now, without brazing, welding or any permanentconnection, the fins are mounted and held securely in proper spacedrelationship upon and in good thermal contact with the exterior of thehousing I.

Moreover, when the housing and the fins are heated in use, the greatercoeificient of thermal expansion of the housing I with relation to thatcoefficient of the fins 2 will cause a tendency for the housing I toexpand more greatly than the fins expand, and this serves further togrip the fins by the expanding housing, and so they are retained firmlyin place. When the housing and fins cool down the fins are still held inplace by the initial expansion-produced distortion of the housing, thatis, by the bowing of the housing intermediate the fins.

I claim as my invention:

1. A heat transfer unit comprising a straightthrough rod-type electricresistance element, two axially bored caps of insulating material, onefor each end of the element, within the bore whereof the respective endsof the elements sheath are received, whereby to support said element,insulation filling the bore, outwardly of the sheaths ends, to preventdischarge from the elements projecting terminals to its sheath, and afinned tubular heat transfer housing surrounding and spaced at allpoints from the element, and mounting said caps as closures for itsopposite ends, to complete a dead air space within said housing.

2. A heat transfer unit comprising a tubular housing, a rod-typeelectric resistance element axially centered within and extending fromend to end of and spaced from said tubular housing to leave an air spacetherebetween, the opposite terminals of the resistance elementprojecting from the opposite ends of the housing, said housing beingformed of metal which possesses a given coefficient of thermalexpansion, dielectric closure means for the ends of said housing, andsupporting the ends of said resistance element, and fins mounted on andin heat transfer engagement with, and projecting outwardly from, saidhousing, said fins being formed of metal which possesses a lowercoefficient of thermal expansion than said housing.

3. A heat transfer unit comprising a fin-encircled tubular housing, arod-type electric resistance element axially centered within andextending through said tubular housing, with its opposite endsprojecting from the opposite ends of the housing, and spaced at allpoints from said resistance element to leave an air space therebetween,said housing being formed of sheet metal soft enough to have beenexpanded under the infiuence of internal pressure in the spaces betweenfins, and the fins which encircle said housing being formed of metalhard enough to resist expansion of the tube and so to be held in placeand in heat exchange relation by the intervening expansion of the tube,the metal of the housing having also a coefiicient of thermal expansionwhich exceeds that coefiicient of the metal of the fins, whereby theirrelative expansion when heated tends to tighten the housing within thefins, and dielectric closure means mounted on the ends of the tube andsupporting the ends of said resistance element.

4. A finned metal heat-transfer tubing which comprises a tube formed ofa metal of a given coefiicient of thermal expansion, and which has asoftness such that it has been expanded by internal pressure, and henceis bowed outwardly intermediate successive fins, a plurality of separatefins of a metal having a lesser coefiicient of thermal expansion,encircling said tube at spaced intervals, and held in place by suchexpansion and bowing of the tube.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 1,475,162 Abbott Nov. 27, 1923 1,668,508 Kettering May 1, 19281,741,217 Winslow Dec. 31, 1929 1,821,702 Freeman Sept. 1, 19311,946,547 Russell et a1 Feb. 13, 1934 1,960,955 Becker May 29, 19342,372,150 Whittaker Mar. 20, 1945- 2,458,189 Morgan Jan. 4, 19492,473,783 Brown June 21, 1949 2,594,465 Lovfald Apr. 2 1952 FOREIGNPATENTS Number Country Date 406,498 Great Britain Mar. 1934

